Friend leukemia and endogenous viruses. I. Production of friend and endogenous C-type particles by mouse cells cultured in vitro.

Within 3 days after infection of $\text{BALB}\text{c}$ 3T3 mouse cells with Friend virus, formation of new virions is detectable by agglutination of ³H]uridine-labeled virus with Con A. Newly infected cells produce a heterogeneous population of particles which includes infectious leukemia virus that is selectively and quantitatively agglutinated by Con A. Particles that are nonagglutinable are released in gradually diminished amounts as cells are serially passaged in vitro. Continued cultivation of the cells results in “spontaneous” production of other particles of density 1.18 that are coproduced with FLV. C-type particles (also of density 1.18) are produced simultaneously by cells not infected with Friend virus. They are morphologically indistinguishable from FLV, bind ferritin Con A, and similarly contain 60–70 S RNA.